For the large part I will be reiterating what others have said. Cleaning and lubing is a good idea if you don't know when it was last done. First off remove the belt and turn the motor shaft by h and there should be very little friction. If it doesn't turn freely cleaning and lubing is necessary. With the belt off the platter should turn for tens of seconds when spun by hand.
The link which JDJX gave looks very good. I like sewing machine oil for lubing the platter bearing and the motor bearings.
Make sure the belt runs centered on the selected part of the capstan. The largest diameter part should be in the middle of the belt.
It is possible that your belt is too thin however that would be unusual as the challenge is to get a belt sufficiently thin. There are many complaints of excessive speed with these machines.
If the belt change arm is not rubbing the belt, the belt is running true, and the friction is reasonable, checking the motor speed is the next step. It should be precisely 1800 RPM on 60 Hz.
This modestly priced optical tach can be used to check the motor:https://www.amazon.com/dp/B01MY4T3NR
It will also check platter speed to the nearest 0.1 RPM.
If the motor speed is correct and the belt is not slipping suspect an excessively thin belt.
Also look for evidence of someone having machined or ground the capstan. It is possible someone did this to get on speed with an excessively thick belt. If the capstan has been altered you may be able to get a replacement from Organ Donor: http://organdonorparts.com/
mrfotch1 wrote:Hello. Just recently acquired my dads old Pioneer turntable. I replaced the belt, but it still runs about 2 RPM slow. Not really a speed inconsistency issue because it stays steady just at a slower speed. Any help trying to fix this issue would be greatly appreciated. Sorry if this was posted before, but I didn't see anything other than inconsistant speed.